Oxide gel based materials (e.g. silica gels) are widely used as catalyst supports for the polymerization of olefins. Silica gel has been used as a support per se or has been used in the form of a cogel or tergel with metals such as chromium and titanium.
Oxide gels (including cogels and tergels) are most often commercially prepared by a gelation from an aqueous system to form a porous hydrogel containing water in its pores. For use in polymerization processes, it is generally necessary to remove the water from the hydrogel. Unfortunately, simple drying cannot be used to remove the water since shrinkage of the desired porosity will occur.
In order to overcome the water removal problem, several techniques have been attempted in the prior art. Many of these methods are discussed in numerous patents assigned to Phillips Petroleum Company. U.S. Pat. Nos. 3,900,457, 4,081,407, 4,152,503 and 4,436,883 discuss the use of azeotropic distillation techniques for removal of water from the gel while avoiding pore collapse. U.S. Pat. No. 4,169,926 discusses a process where the gel is given a one time impregnation with certain organic liquids followed by drying.
While azeotropic distillation has proven to be effective, there are various disadvantages associated with the known processes. For example, certain preferred organic solvents such as those described in U.S. Pat. No. 3,900,457 take a long time to achieve the desired distillation owing to their limited miscibility with water. On the other hand, the organic solvents used in U.S. Pat. Nos. 4,081,407 and 4,152,503 have resulted in more rapid water removal, but these various organic distillation liquids are relatively expensive in the quantities needed for commercial production. For Ti--Cr--SiO.sub.2 systems, these azeotropic processes can result in excessively high melt index capability.
These problems have led to use of alternative techniques such as those described in U.S. Pat. No. 4,169,926 and 4,436,883. While these other techniques may be more economical than azeotropic distillation, the resulting catalyst is generally not as good can be obtained by azeotropic techniques.
Accordingly, there is a need for an economical azeotropic distillation technique for use in production of silica gel based materials for catalyst applications.